Commit 47aa8b6c authored Apr 26, 2013 by Ingo Molnar

nohz: Reduce overhead under high-freq idling patterns



One testbox of mine (Intel Nehalem, 16-way) uses MWAIT for its idle routine,
which apparently can break out of its idle loop rather frequently, with
high frequency.

In that case NO_HZ_FULL=y kernels show high ksoftirqd overhead and constant
context switching, because tick_nohz_stop_sched_tick() will, if
delta_jiffies == 0, mis-identify this as a timer event - activating the
TIMER_SOFTIRQ, which wakes up ksoftirqd.

Fix this by treating delta_jiffies == 0 the same way we treat other short
wakeups, delta_jiffies == 1.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Geoff Levand <geoff@infradead.org>
Cc: Gilad Ben Yossef <gilad@benyossef.com>
Cc: Hakan Akkan <hakanakkan@gmail.com>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 65e709dc

kernel/time/tick-sched.c

+4 −3

Original line number	Diff line number	Diff line
		@@ -565,11 +565,12 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
		delta_jiffies = rcu_delta_jiffies;
		}
		}

		/*
		* Do not stop the tick, if we are only one off
		* or if the cpu is required for rcu
		* Do not stop the tick, if we are only one off (or less)
		* or if the cpu is required for RCU:
		*/
		if (!ts->tick_stopped && delta_jiffies == 1)
		if (!ts->tick_stopped && delta_jiffies <= 1)
		goto out;

		/* Schedule the tick, if we are at least one jiffie off */

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